Crack Tip Opening Angle Measurement Methods and Crack Tunnelling in 2024-T351 Aluminium Alloy

The crack tip opening angle (CTOA) fracture criterion is one of the most promising fracture criterion used to characterise the stable tearing process in metallic materials. Traditional methods used for the experimental characterisation of the CTOA involve accurate identification of the crack tip at each tearing event. Recently alternative methods have been proposed that reduce the necessity of accurately defining the current crack and rely more on the shape of the crack flanks to define the CTOA. In addition, these methods define an 'apparent crack tip', which may be different from the actual surface crack tip and may provide insight into the amount of crack-front tunnelling that is occurring. In the current research, compact tension specimens fabricated from 6.35 mm thick 2024-T351 aluminium alloy plate were evaluated to investigate different CTOA measurement methods and their potential for estimating crack-front tunnelling. In addition to characterizing the CTOA, fatigue marker bands were employed to map the evolution of crack-front tunnelling. The experimental critical CTOA values obtained from the alternative methods were noticeably lower than that obtained from the traditional approach and showed noticeably more scatter. When compared to the experimentally obtained marker bands, the alternative methods indicated limited potential for predicting crack-front tunnelling.